Thermally controlled electric switch



April 4, 0 E. H. TAYLOR 2,503,008

THERMALLY CONTROLLED ELECTRIC swncn Filed Nov. 2, 1 946 2 Sheets-Sheet 1 Inventor 441 .4 vim/Mr A llomey April 1950 E. H. TAYLOR 2,503,008

THERMALLY CONTROLLED ELECTRIC SWITCH Filed Nov. ,2, 1946 2 Sheets-Sheet 2 I nvanlor ric H. Way/or mu, 6. 4M

Allomcy murmur oon'l'aonnnn memo swrrcn Eric Taylor, Burton, England Application November 2, In Great Britain 1946, Serial No. 707.509 November 5, 1945 x I 14 cm (Ci. zoo-13s) The present invention is concerned with thermally controlled electric switches.

It is an important requirement of an electric switch that the contact pressure shall remain as great as possible right-up to the moment of breaking contact, contact shall be as definite as possible so as to avoid sparking and chattering at the contacts.

and that the actual break offrom one stable position v .Various switches have been proposed in which abimetallic member is caused to actuate the switch contacts or some mechanism which controls the switch contacts. These prior proposals have utilised the change in stress or the shape of a bimetallic member upon change of tem-' perature to reduce the contact pressure of a switch until contact can no longer be maintained and/or until the mechanism is caused to actuate the switch. Since the changes in shape or stress of a bimetallic member are comparatively slow, the period during which the contact pressure is being reduced or during which the switch actuating mechanism is functioning is comparatively long with the result that there isalways a tendency to arcing, and chattering of the contacts with consequent damage thereto.

It is an object of the present invention to provide an improved thermally controlled switch which overcomes these disadvantages and avoids the dangers of arcing and chattering oi the contacts. 1

According to the present invention, therefore, there is provided a thermally controlled electric switch comprising a diflerentially stressed sys- 2 has a stable position on each side of an unstable position, through which it passes when moving to the other. If desired such a diflerentially stressed system may in addition comprise one or more magnets, springs, links or levers or the like which serve to urge it into the appropriate stable position of rest-after it has passed through its unstable position or which serves to transmit the motion of said strip or plate in operation.

If desired a strip which may be a bimetallic strip, of substantially U shape may be employed as a difierentially stressed system, means being provided such as a suitable clamp or the like to grip the ends 'of said strip in its original plane. stressing means which serve to stress said ends towards one another are also provided so that when so stressed said strip will be deformed,

- across both its breadth and length, out of its original plane, to one side or the other of said original plane.

A bimetallic member for instance of brass and invar steel is provided with a suitable contact which comprises the moving contact of the switch.

Said contact may, if desired be carried by a spring urged arm, and may be disposed so that said bimetallic member causes or prevents movement thereof according to the changes in the shape and/or stress of said bimetallic memtem as herein defined, adapted to control the movement of a movable contact with respect to a fixed contact, said system incorpora -a bimetallic member which carries -or directly controls said movable contact, said fixed contact constituting an abutment against which said bimetallic member causes or assists'said stressed system to move in a switch operating direction upon a change in temperature altering the shape and/orstress of said bimetallic member, so that said movable contact will be urged initially against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said stressed system through its unstable position. v

It is to be understood that "a differentially stressed system" as used herein means a flexible strip, or plate held in compression bystresses applied across its width so thatthe strip, or plate will be caused to buckle out of its original plane in two directions, e. g. across its breadth and along its length, thus forming a system which Further contacts which may be fixed and which serve to limit the movement of said movable contact in one or both directions are provided in said switch. If desired one of said further contacts may act solely as a stop to the movement of the movable contact when the switch is open. I prefer to provide means whereby said further contacts may be adjusted, as for example, threaded mountings therefor which enable the degree of movement of said movable contact to be varied.

The said bimetallic member may be integral with or part of said differentially stressed system or may be so associated therewith that changes in shape and/or stress of said member cause said movable contact to abut against said fixed contact whilst the reaction in said member causes orassists saidstressed system to move through its dead centre in a switch operating direction from one stressed position to the other, whereupon said movable contact is sharply moved from one switch operating position to the other.

wards one another, in such a case said bimetallic member is preferably in the form of a strip, disposed between the legs of said U, which may be integral with the bridge of said U, and which may carry at the end thereof, remote from said bridge, and said movable contact. If desired the said strip may be curved or otherwise so shaped that the said contacts are displaced from the plane of the said stressed system.

In order that the invention may be well understood preferred embodiments thereof will now be described by way of example only with refence to the accompanying drawings in which- Figure 1 shows in plan view and diagrammatically one form of differentially stressed system.

Figure 2 shows an end elevation of Figure 1 in one position.

Figure 3 shows a section on the line 3-3 of Figure 1.

Figure 4 shows an end elevation of Figure 2 in the alternative position.

Figure 5 shows a side section of Figure 4 on the line 3-3 of Figure 1.

Figure 6 shows an end elevation in part section of one iorm of switch according to the invention.

Figure 7 shows a section on the line 1-1 of Fi ured Figure 8 shows an end elevation of a further form of switch according to th present invention.

Figure 9 is a plan view of the switch shown in Figure 8.

Figure 10 is a side elevation oi a further modification of the invention.

Figure 10a is a front elevational view of Figure 10 showing certain parts broken away; and

Figure 10b is a perspective view of the switch shown in Figures 10 and 10a.

Figures 11, 12 and 13 show three possible modiflcations of the differentially stressed system of Figure 1.

Referring now to Figures 1 to 5 there is shown diagrammatically one form of differentially stressed system which may be embodied in switches according to my invention. A rectangular strip 1 of bimetal, for instance of brass and invar is cut so as to provide three legs 2, 3 and I and a bridge portion 5. A contact 6 is carried by the centre leg 3, and serves as the moving contact of a switch. If now the free ends of the legs 2, and 4 are clamped in their original plane by any suitable means as indicated by the arrows A in Figs. 2 and 4 and are stressed towards one another as indicated by the arrows B in'Figs. 1, 2 and 4, the strip I will be caused to deform in its length and across its breadth, and since the free end of the legs are clamped the member will tend to deflect to one side or the other of its original plane whilst also buckling about the centre line. The centre leg 3 being free to move will take up a position, as shown in Figure 2, where the contact 6 will make contact with a fixed contact I.

It now the brass or high expansion side of the bimetallic strip I is indicated at 9 and the invar or low expansion side at l0, and there is a rise in temperature it will be seen that the increase in length and/or stress in the strip i will tend to move the strip from the one stressed position shown in Fig. 3 through its dead centre or original plane to the other stressed position shown in Fig. 4.

-The centre leg 3 however is in contact with the contact 'I and hence the increase in length and/or stress of the said centre leg 3 will cause the contact pressure between the contacts 6 and I to be increased and will cause said contacts 6 and I to serve as an abutment against which the reaction in the leg 3 will assist in urging the whole strip I towards its dead centre. As the strip 1 passes over its dead centre it will spring, into the other stressed position shown in Figures 4 and 5 and will thus sharply break the contact between the contacts 6 and 1 thus avoidin chattering or sparking. The contact 6 will then abut against the stop 8.

It will be understood that upon an appropriate fall of temperature taking place the reverse procedure will occur andnthe contact between 6 and I will again be restored.

in the instance above described the strip l is so stressed and set that th brass or high expansion side is on the inner curvature of the stressed system which will thus operate on an appropriate rise in temperature takin place. If however it is desired to cause the system to operate on a fall in temperature taking place, the initial stressing will be so carried out that the low expansion or invar side is on the inner curvature of the stressed system.

In Figures 1 to 5 the stressed system comprises a bimetallic strip 1 but it will be appreciated that a stressed system might be made as shown in Figure 11 by employing a substantially U-shaped strip la of steel or other suitable metal and attaching thereto in any suitable manner at the bridge 5 of the said U a bimetallic central arm 3c carrying a movable contact 6. In this case a switch embodying such a stressed system would not be as sensitive as one in which the whole of the strip l is made of bimetal. In this case also the appropriate change in temperature will bring about an increase in the contact pressure and will cause the fixed and moving contact to form an abutment against which the said central bimetallic arm assists in moving the stressed system in a switch operating direction.

Figures 6 and 7 show a switch according to the invention. An insulating block 5 I has a member 12 mounted thereon and a clamping member 13 is located above the member E2, the two outer legs 54, of a bi-metallic strip l8 (as shown in Figure 1) being held between the members'l2 and i3. Said clamping member l3 has deformable portions I6 which are provided with clamping screws l? to enable the portions 16 to stress said legs 54, inwardly as described with respect to Figure 1. A central bimetallic leg [5 which is integral with the said strip it has a contact is thereon. A terminal member 20 is provided so that when the switch is coupled to an electric circuit said terminal member makes contact through the clamping members l2, l3, and the strip 16 with the contact as. In Fig. 6 the movable contact IQ is shown in contact with the fixed contact 2i which is connected to a terminal member 22. It will thus be seen that a compact thermally controlled switch is provided which may be utilised in any system where it is desired to control an electric circuit thermostatically. I

Figures 8 and 9 show an alternative mode oi construction in which the legs 23, 24 of a bimetallic strip of substantially U shape are held in a slot 25 of a pillar 26. Clamping means 27 are provided and a further screwed member 28 is provided to stress said legs together to produce a difierentially stressed system. The centre bimetallic leg 29 which carries the movable swam contact 33 is in this case bent through two obangles so as to displace the said movin: contact 33 and its cooperating fixed contact 3| and stop 32 laterally from the pillar 23. It will be seen that nuts 33 and 34 are provided to enable the relative positions of the fixed contact 3| and the stop 32 to be adjusted. Two suitable terminals 35 and 36 are provided so that a circuit can be completed through the pillar 24 and the central bimetallic leg 29, the movable contact 39 and the fixed contact 3|. It will be seen that in both the switches shown in Figs. 6 and 7 and 8 and 9 the movable and fixed contact will serve as an abutment to cause or assist the stressed system to move in a switch operating direction whilst maintaining or increasing the contact pressure between the said contacts-until the stressed system passes its dead centre and parts said contacts sharply. Figure 9 shows in dotted lines the pwition of the switch when the contact has been broken.

Figures 10,- 10a and 10b show a modification which allows of a degree of adJustment of the operating temperature of the switch. The switch has a body 31 of insulating material. A stressed system 42 is mounted on body 31 and clamped thereon by a clamping member 46 secured to body 31 by screws 41 one of which carries'a terminal 48. Deformable portions provided to enable the system 42 to be stressed.

A terminal 38 secures a flexible metal strip 39, having a contact 40 thereon, to the rear end of the body 31. The said strip 39 is normally biased downwardly from the position shown' in Figure 10 and an adjusting screw 45 is provided which is adapted to urge strip 39 upwards into the position shown. The strip 39 carries a stop member 43 having a lip 44 which overhangs the bimetallic central leg of the stressed system 42 and thus limits the movement of the central leg and its contact M, which is the movable contact of the switch. Thus by screwing up screw 45 and allowing a larger upward movement of contact 40 a greater temperature diiferential is required to operate the switch, whilst by unscrewing screw 45 and allowing strip 39 to assume its normal downwardly inclined position a smaller temperature difierential will cause operation of the switch since the movable contact 49 will be biassed downwardly by the lip 44.

It will be understood that a switch according to the present invention may be used in any form of installation in which thermostatic control is required, that is to say, in heating or refrigerating systems.

Preferred embodiments by way of example of the invention have been described above and it is to be understood that various modifications and variations may be made therein without departing from the scope thereof, thus for instance a dillerentially stressed system comprising a substantially semicircular bimetallic strip lb might be employed as shown in Figure 12, or two bimetallic strips lo and I secured together at an angle might be employed as in Figure 13, in either case a central bimetallic arm 3a would beprovided.

Iclaim:

1. A thermally controlled electric switch comprising a fixed contact and a movable contact, a bimetallic member controlling movement of said movable contact with respect to said fixed contact, a. difierentially stressed switch actuating plate and means for stressing said plate 49 similar to those described with reference to Figure 6 arethrough an unstable position, and

2. A thermally controlled electric switch comprising a fixed contact and a movable contact,

a difierentially stressed switch actuating plate of substantially U-shape, means for-stressing the arms of said plate towards one another to buckle said plate in two directions at right angles,'said plate being mounted for movement between two stable positions through an unstable position; and a bimetallic member disposed between the legs of said plate, said member being operatively connected to said plate and carrying said movable contact, said fixed contact constituting an abutment against reacts and causes operating direction when a predetermined temperature change being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of'said plate through its unstable position,

3.. A thermally controlled electric switch as set forth in claim 2 in which said switch actuating plate is a bimetallic plate; said bimetallic member being integral therewith.

4. A thermally controlled electric switch comprising a fixed contact and a movable contact,

a diilerentially stressed switch actuatin plate of substantially U-shape, means for gripping the ends of the legs of said plate and stressing them towards each other, so as to buckle said plate in two directions at right angles, the end of said plate remote from said stressing means being movable between .two stable positions member connected to the movable end of said plate and carrying said movable contact. said fixed contact constituting an abutment against which said bimetallic member reacts and causes said movable end of saidplate to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said movable end I through its unstable position- 5. A thermally controlled electric switch comprising a fixed contact and a movable contact,

a bimetallic switch actuating plate slotted at one end into three legs, means for, stressing'two of said legs towards one another so as-to buckle said plate in two directions at right angles to each other, the end of said plate remote from said legs being movable between two stable. positions through an unstable'position, said movable contact being carried by the third of. said legs, said fixed contact constituting an. abutment against which said third leg reacts and causes said plate to move in a switch operating direction upon a predetermined temperature change occurring, 'said third leg urging said movable contact against said fixed contact in a direcas to buckle a in we direc-' which said bimetallic member" said plate to move in a switch occurs, said movable contact a bimetallic 7 -tion opposite to that in which it is ultimately moved by the movement of said plate through its unstable position.

6 A thermally controlled electric switch as set forth in claim in which said plate is of substantially rectangular shape, said movable contact being carried by the center of said three legs.

7. A thermally controlled electric switch comprising a fixed contact and a movable contact, a diflerentially stressed switch actuating plate having two legs, means for gripping the ends of said legs, deformable portions integral with said gripping means for stressing the ends oi! legs towards one another so as to buckle said plate in two directions at right angles, the end 0! said plate remote from said gripping means being movabl between two stable position through an unstable position, a bimetallic member operatively connected to said plate and disposed between said legs, said bimetallic member controlling the movement of said movable contact, said fixed contact constituting an abutment against which said bimetallic member reacts and causes said plate to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by the movement of said plate through its unstable position.

8. A thermally controlled electric switch as set forth in claim 7 in which said switch actuating plate is a bimetallic plate, said bimetallic member being integral therewith.

9. A thermally controlled electric switch comprising an insulating base, clamping members mounted on said base, a fixed contact adjustably secured to one of said clamping members, a switch actuating plate of substantially U-shape the ends of the legs of which are gripped between the said clamping members, means for stressing said legs towards one another so as to buckle the plate along its length and across its width, the end 01 said plate remote from said stressing means being free for movement between two stable positions through an unstable position, a

bimetallic member operatively associated with i said remote end of said plate, and a movable contact carried by said bimetallic member, said fixed contact constituting an abutment against which said bimetallic member react and causes said plate to move in a switch operating direction when a predetermined temperature change occurs, said movable contact being urged against said fixed contact in a direction opposite to that in which it is ultimately moved by movement of said plate through its unstable position.

10. A thermally controlled switch as set forth in claim 2 in which said bimetallic member and movable contact are laterally displaced from the plane of said switch actuating plate.

11. A thermally controlled electric switch as set forth in claim 5 in which said third leg carrying said movable contact is bent through two obtuse angles so as to displace said leg and said contact laterally with respect to said switch actuating plate.

12. A thermally controlled switch as set forth in claim 2, comprising an adjustable mounting for said fixed contact whereby the operating temperature of the switch may be regulated by varying the initial pressure between said fixed and movable contacts.

13. A thermally controlled switch comprising a fixed contact and a movable contact, a differentially stressed switch actuating plate of substantially U-shape, a slotted gripping member securing the ends of the legs of said plate, means for clamping said legs in said slot and applying stress across the width of said plate so as to buckle the plate along its length and across its width, the end of said plate remote from said gripping member being movable between two stable positions through an unstable position, and a bimetallic member carrying said movable contact and connected to said plate between the legs thereof, said fixed contact constituting an abutwent against which said bimetallic member reacts and causes said plate to move in a switch operatin direction when a predetermined temperature change occurs, said movable contact bein urged against said fixed contact in a direction opposite to that in which it i ultimately moved by the movement f said plate through its unstable position.

14. A thermally controlled electric switch comprising a fixed contact and a movable contact, a bimetallic switch actuating plate having a pair of parallel legs, means for gripping the ends of said legs and stressing them towards each other so as to buckle said plate in two direction at right angles, the end of said plate remote from said stressing means being movable between two stable positions through an unstable position, a third arm extending from the free end of said plate generally in the same direction as said pair of arms and carrying said movable contact, said fixed contact being mounted to engage said movable contact in one stable position of said plate, and said fixed contact constituting an abutment against which said third arm reacts when a predetermined temperature change occurs and causes the movable end of said plate to move into its other stable position.

ERIC I-IARDMAN TAYLOR.

REFERENCES CKTED UNITED STATES PATENTS Name Date Dubilier June 30, 1942 Number 

